As is well known, superconducting circuitry requires maintenance at low temperatures in order to retain superconductivity characteristics. If coolant is used it must be transferred from a storage dewar to the superconducting circuity. An example of a suitable coolant is liquid helium. For ease of handling, it is desirable that the transfer line be a flexible line.
It is advantageous for the transfer line to deliver liquid helium to the superconducting circuitry at a low flow rate in order to keep to a minimum the quantity of liquid helium used. A flexible liquid helium transfer line has previously been used which has a vacuum jacket of stainless steel flexible tubing. This known transfer line also has multiple layers of superinsulation surrounding a delivery tube.
The standard liquid helium transfer line described above does not work well at low-flow rates (below two liquid liters per hour), but is instead designed for fast transfer of bulk quantities from one storage dewar to another. One source of inefficiency at low-flow rates is that the external heat radiation impinging on the delivery tube is generated at room temperature, and is only partially shielded from the delivery tube due to imperfect insulation by the multi-layer insulation. Another source of inefficiency is the heat which is conducted from the vacuum jacket by supports which center and space the delivery tube from contact with the vacuum jacket. These inefficiencies cause the minimum flow rate necessary to achieve proper cooling of the superconducting circuitry to vary with the positioning of the flexible line.
Due to the above inefficiencies, using the standard transfer line to cool superconducting circuitry leads to the problems of high liquid helium consumption and high operating costs, since a higher rate of flow of liquid helium is needed to make up for the losses in the transfer line.